Recently, composites in which carbon fibers, aromatic polyamide fibers or the like are used as reinforcement have been frequently used as structural materials for aircraft and the like, based on their high specific strength and high specific rigidity.
Composites which have excellent thermal resistance, mechanical characteristics, dimensional stability, chemical resistance and weather resistance have been provided by combining aromatic glycidylamine epoxy resins as the matrix resin with a diaminodiphenylsulfone as a curing agent.
Though such composites prepared from epoxy resin-based prepregs show good performance, it has been found that they have inferior toughness and impact strength because of the short shelf-life of the prepreg and the low elongation and, therefore, brittleness of the matrix resin.
Especially, an improvement of the impact strength of the composite without reducing the thermal resistance thereof has been regarded as an important object in the case of the use of such composites as primary structural materials in aircraft which are apt to receive impact from the outside, for instance, by pebbles bounced up at the time of take-off or landing, from tools dropped by mistake during maintenance and repair and the like.
In order to increase the high impact strength of a composite, it is important not only to improve the elongation of a reinforcement such as carbon fibers but also to increase the toughness of a matrix resin. For this reason, a number of attempts have been made to improve matrix resins.
In order to improve the toughness of matrix resins, several methods have been proposed such as a method in which an epoxy resin is mixed with a rubber component and a method in which an epoxy resin is mixed with a high molecular weight component. When an epoxy resin is mixed with a rubber component, the toughness and impact strength of the resulting molded product can be improved, but its thermal resistance and mechanical characteristics are reduced. Because of this, the mixing ratio of the rubber component is limited, sometimes to a low mixing ratio depending on the use of the product, thus resulting in insufficient improvement.
In order to mix an epoxy resin with a thermoplastic resin as a high molecular weight component, a thermoplastic resin is dissolved in the epoxy resin at high temperature, or a thermoplastic resin is dissolved in a solvent and then the epoxy resin is added to the solution. When a thermoplastic resin is dissolved in an epoxy resin at a high temperature, the viscosity of the resin mixture increases and, at the same time, its tackiness decreases, thus resulting in markedly poor handling ability. Mixing of these resins using a solvent also has disadvantages such as the problem of removing the solvent after mixing, complex preparation steps and a decrease in thermal resistance due to a small amount of remaining solvent.
As a consequence, conventional resin compositions prepared by mixing an epoxy resin with a small amount of a rubber component or a high molecular weight component provide a composite with a poor impact strength-improving effect though the decrease in thermal resistance is not so great.
Other types of resin compositions used as prepregs have been disclosed, for instance, in JP-A-61-250021, JP-A-62-57417 and JP-A-63-162732 (corresponding to U.S. Pat. No. 5,028,478), in which a thermoplastic resin is dispersed in and mixed with an epoxy resin to improve toughness (impact strength) of a composite, but with insufficient effect in terms of impact strength of the composite. (The term "JP-A" as used herein means an "unexamined published Japanese Patent Application".)
In addition, in these resin compositions, a diaminodiphenylsulfone compound is used as an epoxy resin-curing agent in view of the thermal resistance of the resulting composite. When such a compound is used as a curing agent, the resulting resin composition has a short shelf-Life of about 2 to 3 weeks at room temperature (about 23.degree. C.), thus resulting in a handling problem.
Accordingly, this invention contemplates overcoming the above-mentioned problems involved in the prior art.